"On the Linux kernel mailing list, Willy Tarreau has announced that there will be no more releases for version 2.4 of the Linux kernel. Tarreau, who is responsible for maintaining the Linux 2.4 kernel, said that 'few people' still use this version. According to him, these users are better served by a central Git repository that collects bug fixes and that he has now created such a repository for them." I remember when 2.4.0 was released... Proper USB support! Them were the days.

RHEL 4 went EOL (technically End of Production according to https://access.redhat.com/support/policy/updates/errata/) on February 29, 2012. It's now only receiving critical impact security fixes if you pay for a special Extended Life Cycle Support subscription above and beyond the normal RHEL subscription.

Both CentOS and Scientific Linux ended their 4.x lines on February 29. So, for all intents and purposes, RHEL 4 is publicly End Of Life.